#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define MAXN (300005)
ll N,M;
ll A[MAXN], B[MAXN];
bool bs(ll mid){
ll extra = 0;
ll arr[N];
bool done[N];
memset(done,0,sizeof(done));
for(ll i = 0;i < N;i++){
arr[i] = mid;
}
for(ll i = 0;i < N;i++){
ll minimum = mid / A[i];
if(minimum * A[i] < mid) minimum++;
if(minimum <= M){
extra += (M - minimum);
done[i] = 1;
arr[i] = 0;
}else{
arr[i] -= (M * A[i]);
}
}
for(ll i = 0;i < N;i++){
if(!done[i]){
ll minimum = arr[i] / B[i];
if(minimum * B[i] < arr[i]) minimum++;
if(minimum <= extra){
extra -= minimum;
done[i] = 1;
arr[i] = 0;
}else{
return false;
}
}
}
return true;
}
int main() {
ios_base::sync_with_stdio(false);cin.tie(0);
cin>>N>>M;
for(ll i = 0;i < N;i++){
cin>>A[i];
}
for(ll i = 0;i < N;i++){
cin>>B[i];
A[i] = max(A[i],B[i]);
}
ll high = 2e18 + 5;
ll low = 0;
while(high - low > 1){
ll mid = (high + low) / 2;
if(bs(mid)){
low = mid;
}else{
high = mid;
}
}
cout<<low<<'\n';
}
